Initially tubby uncle, rather podgy, slightly thinner (5)
I believe the answer is:
turps
'thinner' is the definition.
(I've seen this before)
'initially tubby uncle rather podgy slightly' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'tubby uncle rather podgy slightly' is 'TURPS'.
(Other definitions for turps that I've seen before include "Paint thinner or solvent (abbr)" , "cleaning agent" , "Paint cleaner" , "Paintbrush cleaner" , "Paint/varnish ingredient" .)